Sean Penn Goofs Off with Charlize Theron's Son on Grocery Run

01/23/2014 at 03:00 PM EST

Sean Penn Goofs Off with Charlize Theron's Son on Grocery Run
When mom's not looking, her boys are going to play!

Sean Penn was caught pulling a goofy face on Charlize Theron's son Jackson, 2, while the trio shopped for groceries Wednesday.

"It was adorable to watch how sweet Sean was with Jackson. It was obvious that they are still getting to know each other and Jackson kept making sure that his mom stayed close," an onlooker tells PEOPLE.

Penn, who brought reusable bags on the Whole Foods run, is no stranger to entertaining kids: He has two children with ex Robin Wright.

Theron and Penn's courtship first made waves (literally) over the holidays, when the pair spent the New Year surfing in Hawaii. Their romance has proven to be more than a vacation fling, as Theron, 38, and Penn, 53, have been spotted together all over L.A.

Though they have yet to publicly confirm their relationship, an eyewitness told PEOPLE the couple engaged in plenty of PDA at Penn's Help Haiti Home gala Jan. 11.

And, while picking up their goods on Wednesday, "Sean and Charlize looked very happy together. They were flirty and seemed to enjoy their day together," said the onlooker.

Hopefully little Jackson, whom Theron adopted in 2012, warms up to Penn, too.

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